Applicants' invention relates to charging a battery in a portable electronic apparatus, e.g., a mobile radio communication system such as a portable cellular telephone, a cordless phone, a pager, etc. The invention more particularly relates to methods and apparatus for delivering a constant current from a charger to a battery in a cellular phone using control logic in the phone.
Mobile communication technology has evolved from early analog systems, e.g., the NMT450 and NMT900 systems in Scandinavia, the AMPS system in the U.S., and the TACS and ETACS services in the United Kingdom, to more sophisticated digital systems, e.g., the Global System for Mobile Communications (GSM) in Europe and the American Digital Cellular System (ADC) in North America. Modern digital mobile communication systems are capable of voice transmission as well as other digital services, facsimile transmissions, and short message services.
Mobile communication devices like cellular phones should preferably be small, lightweight, and inexpensive, yet exhibit good battery performance and be capable of providing many services. The communication device should not have any unnecessary external connectors, which are expensive and increase the size and weight of the device. To ensure proper operation of a battery-powered communication device, it is critical that the battery is correctly charged (and discharged). Preferably, the communication device should be operable while the battery is charging, so that operation need not be repeatedly interrupted to recharge the battery.
Among the known techniques for charging a battery in a portable communication device is connecting a battery charger to the battery through the communication device. This may be done by placing a cellular phone in a click-in battery-charging holder in a car. A switch is often included that operates only to switch off the charging current when the battery is fully charged. Charge is delivered to the battery only when the cellular phone is placed in the click-in holder. Since charge is depleted during a call, the cellular phone must be put back in the click-holder to recharge the battery and interruption may result. Furthermore, these devices do not distinguish between different types of batteries, so that a cellular phone may be erroneously connected to an inappropriate charger, resulting in incorrect charging and possible damage to the battery.
To correct the problem of incorrect charging, U.S. Pat. Nos. 5,164,652 and 5,237,257, both to Johnson et al., disclose devices for determining the type of battery connected to the charger and adjusting charging parameters accordingly so that the battery is correctly charged. To prevent improperly discharging a battery, U.S. Pat. No. 5,177,426 to Nakanishi et al. discloses a device using circuitry in the charger to stop current from flowing from the battery to the phone. Such devices typically employ logic circuitry that is part of the charger, which increases the cost of the charger and the overall cost of the cellular system.
Other known battery-charging devices include "intelligent batteries" that store information relating to battery condition, number of charges, and number of discharges in circuitry included in the battery pack. Also known are "intelligent chargers" that sense the condition of a connected battery and charge the battery accordingly.
Such devices are undesirable in portable cellular phones because they add components to the battery and charger that increase the overall cost and weight of the phone. It would be preferable to keep all necessary logic circuitry outside of the battery and charger, where it might be more cost effectively integrated with already existing circuitry.
Yet another device for charging a battery is disclosed in European Patent Publication No. EP 0 450 783 A2 by Martensson that shows control logic in a phone for controlling an external charger. In this device, there must be two-way communication between the phone and the external charger, which requires a control wire and results in a complicated battery charging process.